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  • 1
    Electronic Resource
    Electronic Resource
    Picosecond measurements of relaxation of internal modes in crystalline benzene as a function of temperature (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2967-2973 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relaxation rates of four internal vibrational levels of benzene crystal have been measured by time-resolved coherent anti-Stokes Raman scattering (CARS) at different temperatures. The important information on the dephasing mechanisms provided by the experiment has been supported by anharmonic calculations, which include the full contribution of the density of phonon states, by utilizing average coupling coefficients. Different behaviors have been observed for the different modes considered. Three of the four vibrations (v1,v6, and v10) show linewidths that increase linearly in the classical regime with temperature; the experimental evidence for the important role played by three-phonon processes (driven by cubic anharmonicity) is confirmed by the calculations, which give a quantitative agreement with the observed linewidths. For the above-mentioned vibrations the role of pure dephasing results in a minor contribution, while the effect of isotopic impurities is important in determining the low temperature relaxation rate. On the other hand, the linewidth of v9 increases quadratically with T: Both decay processes of high order and pure dephasing may be responsible for such a behavior. This ambiguity cannot be overcome by our calculations, since these do not include the effect of high order (mainly quartic) anharmonic terms. Finally, the analysis of the decay mechanisms as predicted by the calculation shows that the anharmonic coefficients may differ from mode to mode: the relaxation mechanism is highly mode selective, and its efficiency depends greatly on the nature of the molecular normal coordinates involved in the process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458884
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  • 2
    Electronic Resource
    Electronic Resource
    Computer simulation of the dynamics of the plastic phase of succinonitrile (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 679-685 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The complex dynamics of the plastic phase of crystalline succinonitrile CN–CH2–CH2–CN, involving trans–gauche isomerization as well as molecular reorientations along the diagonals of the cubic unit cell, is investigated by means of molecular dynamics simulation. A simple molecular model in which the end CN groups and the CH2 groups are represented by single pseudoatoms is proposed. The model includes, in addition to the external degrees of freedom, three degrees of internal freedom described by two bending and a torsion coordinate. The intermolecular interactions are treated in terms of Lennard-Jones potentials. Force constants are used for the bending coordinates and the intramolecular torsion is described by a two minima potential function with three parameters. The dynamical properties of the crystal are investigated by calculation of the time dependence of single molecule autocorrelation functions. It is found that the most suitable autocorrelation functions are those involving the torsion and the orientation of the central C–C bond and of the vector joining the two CN end atoms. Three distinct regimes are observed for the orientational relaxation of the model system: (a) a very fast relaxation with a time constant τ〈1 ps due to the overdamped librations and torsional vibrations; (b) a fast decay with τ≈3–4 ps due to the spinning motion of the trans conformers around the long inertia axis; (c) a slow diffusive regime with τ≈20–30 ps due to the molecular tumbling motions and to the trans–gauche isomerization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461418
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  • 3
    Electronic Resource
    Electronic Resource
    Orientational dynamics on glassformer 2 [Ca(NO3)2]⋅3[KNO3]: A study by transient optical Kerr effect (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 4892-4896 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The fast orientational dynamics of molten 2[Ca(NO3)2]⋅3[KNO3] is investigated by time- resolved optical Kerr effect (OKE) with subpicosecond resolution in the temperature range 380–620 K. Apart from the instantaneous electronic response and a very fast decay, the OKE intensity profiles show clearly the presence of a nonexponential decay. The simplest function which fits the experimental data is a biexponential with time constants ranging from 120 to 15 and 10 to 3 ps, respectively. The results are discussed in terms of an integrated correlation time τc that does not show a diverging behavior approaching the glass transition. At high temperatures τc follows a typical Arrhenius law with activation energy of 1.5 kcal/M. Different possible interpretations of the observed behavior (α–β bifurcation, domain size effect) are discussed. The main conclusion is that rotational motions are decoupled from translational ones even for temperatures well above the liquid–glass transition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464944
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular dynamics of β-carotene in solution by resonance enhanced optical Kerr effect (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 9537-9543 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The orientational dynamics of β-carotene in n-alkane solutions is investigated by resonance enhanced optical Kerr effect. By use of this spectroscopic technique, it is possible to selectively investigate the relaxation of a probe molecule at a concentration level low enough to allow the observation of the averaged single-molecule dynamics. For delay times longer than ∼20 ps all solutions show a single exponential decay, with a time constant depending on the viscosity, that is ascribed to the β-carotene orientational relaxation. The dependence on viscosity of the measured relaxation times is compared with the predictions of different models. The purely hydrodynamic theories overestimate, by far, the solute effective volume and hence its orientational relaxation time; a much better agreement is obtained from two quasihydrodynamic models. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468769
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  • 5
    Electronic Resource
    Electronic Resource
    Simulated structure, dynamics, and vibrational spectra of liquid benzene (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6851-6863 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A classical molecular dynamics simulation of liquid benzene is performed, using a potential model which allows for full molecular flexibility. The short range intermolecular radial distribution function is on average reminiscent of the crystalline structure, although practically no preferential orientation can be found for the molecules in the first coordination shell. The average cage lifetime and its vibrational dynamics are obtained from appropriate time correlation functions. The intramolecular vibrations are investigated by calculating the vibrational density of states and the infrared and Raman spectra, achieving an excellent agreement with the experimental data. Finally, the dephasing of the ν1(A1g) ring breathing mode and of the ν6(E2g) in-plane bending mode is analyzed on the basis of the Kubo dephasing function. For ν1 mode the Kubo correlation time of 516 fs agrees with the experimental value, and is consistent with a relaxation mechanism involving the cage reorganization. In contrast, ν6 has a practically pure Lorentzian line shape, with a width of 7.16 cm−1 in perfect agreement with the experimental value of 7.2 cm−1. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1290729
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  • 6
    Electronic Resource
    Electronic Resource
    Transient Absorption and Vibrational Relaxation Dynamics of the Lowest Excited Singlet State of Pyrene in Solution (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 7439-7445 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100019a029
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  • 7
    Electronic Resource
    Electronic Resource
    Temperature-dependent decay of vibrational excitons in dipotassium sulfate crystal measured by picosecond time-resolved CARS (1988)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 92 (1988), S. 983-988 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100315a025
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  • 8
    Electronic Resource
    Electronic Resource
    Electrical response in chemical potential equalization schemes (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 8569-8575 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we compare the polarization response given by two different chemical potential equalization schemes to be applied to molecular dynamics simulations: the standard fluctuating point charge model (FQ) and the atom–atom charge transfer model (AACT). We have tested the transferability of FQ and AACT parameters, fitted to the polarizability of small size alkanes and polyenes, to large size homologues. We show that the FQ scheme is not adequate for the n-alkanes as it strongly overestimates the polarizability tensor components as the number of carbon atoms increases. The FQ approach has been found more predictive for highly conjugated systems like polyenes, although still unsatisfactory. The AACT parameters tuned on ethane are instead perfectly transferable to alkanes of any length and conformation. The AACT scheme satisfactorily reproduces the polarization response also for highly conjugated systems. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480198
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  • 9
    Electronic Resource
    Electronic Resource
    Calculation of optical spectra in liquid methanol using molecular dynamics and the chemical potential equalization method (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 4218-4229 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We apply the chemical potential equalization (CPE) method to the calculation of the optical spectra in liquid methanol at 298 K and normal pressure. The configurations of the liquid are obtained by conventional molecular dynamics (MD) using a completely flexible all-atoms model. The infrared and Raman spectra are computed a posteriori using a CPE parametrization of methanol calibrated to reproduce the electronic properties of the isolated molecule evaluated with accurate ab initio calculations. The MD/CPE method reproduces correctly the optical spectra in the region of the intermolecular motions. The spectra are discussed and interpreted on the basis of hydrogen bonding structure and dynamics. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479720
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  • 10
    Electronic Resource
    Electronic Resource
    Diffusive and oscillatory dynamics of liquid iodobenzene measured by femtosecond optical Kerr effect (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 8653-8662 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of liquid iodobenzene are studied by means of time resolved heterodyne detected optical Kerr effect in a wide temperature range (263–371 K). In the picosecond regime the relaxation is characterized by a biexponential decay, attributed to the rotational diffusion of an anisotropic rotator. The temperature dependence of the two relaxation times agrees only in part with the prediction of the hydrodynamic theory. The subpicosecond dynamics is essentially oscillatory in nature; the Raman spectra obtained by Fourier transform of the time domain data show the contribution of two intramolecular low-frequency vibrations, and that of the intermolecular dynamics. The intermolecular spectra at different temperatures are interpreted on the basis of the Brownian oscillator model, and consist of the superposition of overdamped and underdamped modes. The intermolecular spectrum of the liquid shows a close resemblance with the low-frequency Raman spectrum of crystalline iodobenzene, and suggests an interpretation in terms of collective dynamics with coupling of librational and translational oscillations. The evolution with temperature of the spectra indicates that at high temperature the time-independent picture of the intermolecular modes is not adequate. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478773
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